1. Field of the Invention
The present invention relates to the field of signal transmission using optical orthogonal frequency division multiplexing (OOFDM) transceivers and to a synchronisation method for improving the receiving process.
2. Description of the Related Art
It is known to use optical orthogonal frequency division multiplexing (OFDM) modulation technique in order to reduce optical modal dispersion in multimode fibre (MMF) transmission links, as disclosed for example in Jolley et al. (N. E. Jolley, H. Kee, R. Richard, J. Tang, K. Cordina, presented at the National Fibre Optical Fibre Engineers Conf., Annaheim, Calif., Mar. 11, 2005, Paper OFP3). It offers the advantages of great resistance to dispersion impairments, efficient use of channel spectral characteristics, cost-effectiveness due to full use of mature digital signal processing (DSP), dynamic provision of hybrid bandwidth allocation in both the frequency and time domains, and significant reduction in optical network complexity.
It can also be used advantageously for dispersion compensation and spectral efficiency in single mode fibre (SMF)-based long distance transmission systems such as described for example by Lowery et al. (A. J. Lowery, L. Du, J. Armstrong, presented at the National Fibre Optical Fibre Engineers Conf., Annaheim, Calif., Mar. 5, 2006, paper PDP39) or by Djordjevic and Vasic (I. B. Djordjevic and B. Vasic, in Opt. express, 14, no 9, 37673775, 2006).
The transmission performances of OOFDM have been studied and reported for all the optical network scenarios including long-haul systems such as described for example in Masuda et al. (H. Masuda, E. Yamazaki, A. Sano, T. Yoshimatsu, T. Kobayashi, E. Yoshida, Y. Miyamoto, S. Matsuoka, Y. Takatori, M. Mizoguchi, K. Okada, K. Hagimoto, T. Yamada, and S. Kamei, “13.5-Tb/s (135×111-Gb/s/ch) no-guard-interval coherent OFDM transmission over 6248 km using SNR maximized second-order DRA in the extended L-band,” Optical Fibre Communication/National Fibre Optic Engineers Conference (OFC/NFOEC), (OSA, 2009), Paper PDPB5) or in Schmidt et al. (B. J. C. Schmidt, Z. Zan, L. B. Du, and A. J. Lowery, “100 Gbit/s transmission using single-band direct-detection optical OFDM,” Optical Fibre Communication/National Fibre Optic Engineers Conference (OFC/NFOEC), (OSA, 2009), Paper PDPC3) or metropolitan area networks such as described for example in Duong et al. (T. Duong, N. Genay, P. Chanclou, B. Charbonnier, A. Pizzinat, and R. Brenot, “Experimental demonstration of 10 Gbit/s for upstream transmission by remote modulation of 1 GHz RSOA using Adaptively Modulated Optical OFDM for WDM-PON single fiber architecture,” European Conference on Optical Communication (ECOC), (Brussels, Belgium, 2008), PD paper Th.3.F.1) or in Chow et al. (C.-W. Chow, C.-H. Yeh, C.-H. Wang, F.-Y. Shih, C.-L. Pan and S. Chi, “WDM extended reach passive optical networks using OFDM-QAM,” Optics Express, 16, 12096-12101, July 2008), or local area networks such as described for example in Qian et al. (D. Qian, N. Cvijetic, J. Hu, and T. Wang, “108 Gb/s OFDMA-PON with polarization multiplexing and direct-detection,” Optical Fibre Communication/National Fibre Optic Engineers Conference (OFC/NFOEC), (OSA, 2009), Paper PDPD5) or in Yang et al. (H. Yang, S. C. J. Lee, E. Tangdiongga, F. Breyer, S. Randel, and A. M. J. Koonen, “40-Gb/s transmission over 100 m graded-index plastic optical fibre based on discrete multitone modulation,” Optical Fibre Communication/National Fibre Optic Engineers Conference (OFC/NFOEC), (OSA, 2009), Paper PDPD8).
All prior art existing systems were based on transmission of OOFDM signals originating from arbitrary waveform generators (AWG) using off-line signal processing-generated waveforms. At the receiver, the transmitted OOFDM signals were captured by digital storage oscilloscopes (DSO) and the captured OOFDM symbols were processed off-line to recover the received data. Such off-line signal processing approaches did not consider the limitations imposed by the precision and speed of practical DSP hardware that are required for insuring real-time transmission.
Other work, described for example in WO98/19410 or EP-A-840485, or U.S. Pat. No. 5,953,311 disclosed a method for determining the boundaries of guard intervals of data symbols received in a coded orthogonal frequency division multiplexed (OFDM) signal. In that method, temporal signals separated by an interval of an active interval of a data symbol were associated in pairs and difference signals obtained. The dispersion of a first and second comparison blocks of difference signal were compared wherein the second comparison block was displaced from the first comparison block by n samples.
U.S. Pat. No. 6,359,938 and US2003/0142764 disclosed a single chip implementation of a digital receiver for multicarrier signals transmitted by OFDM. It included an improved FFT window synchronisation circuit coupled to the re-sampling circuit for locating the boundary of the guard interval transmitted with the active frame of the signal.
In US2004/0208269, the synchronisation in the receiver was carried out by separately taking into account both the amplitude and phase differences, thereby providing a clear distinction between the periods during which guard period samples are process and those during which they are not.
In U.S. Pat. No. 5,555,833, the signals were formatted in symbol blocks wherein each block comprised redundant information. It also included means for delaying the symbol blocks and for subtracting said delayed symbol block from the corresponding symbol block. The difference signal was then used to control a loop comprising a local oscillator operating at the clock frequency.
EP-A-1296493 disclosed a synchronisation apparatus that comprised                a) a signal magnitude calculator for calculating the magnitude of an applied input complex signal during time T1;        b) a first delay unit for delaying the signal received from the calculator;        c) a first adding unit for subtracting delayed signal b) from input complex signal;        d) an absolute value calculator applied to the difference of c) to provide absolute value signal;        e) a second delay unit for delaying absolute value signal d)        f) a second adding unit for subtracting delayed signal e) from absolute value signal d);        g) a moving window sum unit for calculating the sum of the signals received during time T2;        h) a searching unit for comparing values of the accumulating unit during time T1 and searching for predetermined point;        i) a guard interval removing unit using searched position h).        
In GB-A-2353680, synchronisation was achieved using a frame synchronisation pulse generated by deriving absolute values of successive complex samples of the OFDM symbol, determining the difference between these values and other values separated by a period representing the useful part of the OFDM symbol, integrating the differences over a plurality of symbols and determining the sample position of the point at which said integrated difference values changed substantially.
US2005/0276340 detected the symbol boundary timing in the receiver of a multicarrier system by:                receiving a series of received training signals over a wire-based channel;        storing at least 3 of these series to a buffer;        determining difference values for a pair of consecutive received training signals stored in the buffer;        selecting one of the difference values;        determining the received symbol boundary timing based on the selected difference value.        
The known systems have been improved by introducing signal modulation technique known as adaptively modulated optical OFDM (AMOOFDM), offering advantages such as:                flexibility, robustness and optimal transmission performance;        efficient use of spectral characteristics of transmission links; individual subcarriers within a symbol can be modified according to needs in the frequency domain;        use of existing multimode fibres;        low installation and maintenance cost.        
These have been described and discussed for example in Tang et al. (J. Tang, P. M. Lane and K. A. Shore in IEEE Photon. Technol. Lett, 18, no 1, 205-207, 2006 and in J. Lightw. Technol., 24, no 1, 429-441, 2006) or in Tang and Shore (J. Tang and K. A. Shore, in J. Lightw. Technol., 24, no 6, 2318-2327, 2006). Additional aspects such as                the impact of signal quantisation and clipping effect related to analogue to digital conversion (ADC) and determination of optimal ADC parameters;        maximisation of transmission performance;have been described in Tang and Shore (J. Tang and K. A. Shore, in J. Lightw. Technol., 25, no 3, 787-798, 2007).        
In order to implement real-time OOFDM transceivers, there is a need to develop advanced high-speed signal processing algorithms with adequate complexity.